International Conference – Innovative technologies for joining advanced materials – tima09

High-precision Plasma Arc Welding of Thin Stainless Steel Plates J. Dobranszky Research Group for Metals Technology of the Hungarian Academy of Sciences, Goldmann ter 3, 1111 Budapest, Hungary E-mail: [email protected]

Abstract Pulsed non-transferred plasma arc welding was used for precision welding of the longitudinal and circular seams of thin-walled stainless steel cylindrical housings. Depending on the plate thickness and joint type, the welding was done by adding filler wire or without them. These papers presents evaluation of technological aspects from the point of view of the formation of weld defects, and evaluate the principal role of the tungsten electrode.

Introduction A well-known and widely used technology of the automatic welding at thin plates is the plasma arc welding. This method/procedure/treatment/process expressly can be used for welding of butt welds of the austenitic stainless steels, as thanked to the concentrated heat source of the well focused plasma arc and plasma beam the input energy can be low in the heat affected zone and that is why decreases the possibility of harmful precipitations for corrosion resistance [1]. At welding of seam weld of the cylindrical housing the electrode gap in the plasma torch is usually prescribed with decimal millimetre precision/accuracy. Setting and control of it is a very important part of the technology [2]. Also important to supervise before the welding, the optimal place of the anode spot, for example with targeting laser. Mistakable change of torch-workpiece distance can influence the electrode gap also [3-4]. Any deviance from the optimum effects to the temperature of plasma, and so effects on weld penetration and burn-through.

metal, circular welds with filler metal by pulsed transferred plasma arc welding. At the ends of the seam welds there were put backings made from brass, edges of the plate all the time were pressing by hold-down clamps (Fig. 1). The aim of this work is to present those experiences that concern the importance of fine details of the welding technology.

Welding of longitudinal seams Due to our examinations the principal failure of the longitudinal seams are the burn-throughs. Most part of the burnthroughs at longitudinal seams mostly form at the starting section of the seam. Sometimes burn-through is caused, because the cylindrical housing can not bear the heat shock of the plasma stepping over to it from the tabs (Fig. 1) of each end of the seam, when can not give that to the copper backing laying below, as it is typical mistake that the edges do not lay on it perfectly. It happens because in the entry zone of plasma the rolling leave unevenness and the strong hammering for evenness deform backings at the end of the seam welds. In those places, where the cylinder does not lay on the bolster plate the root of the weld is melted more than it is normal, but in the laying places the root is cannot melt through, and it can cause several problems (Fig. 2). At longitudinal seams often happens that some parts of the joint touch with the backing bar; in these places unexpectedly increases the heat conduction, and this significantly reduces the weld penetration.

It happens that there is no prescription for angular offset of the torch, however it effects considerably the tilt with a few degrees back or forward of that, which so useful to optimize by experimental way. Among the welding parameters sometimes it is not suitable the shielding gas pre-flow time, which is practically to choose at least 2 second: its role the effective protection of the nozzle and the electrode and the welding material [5]. In this article there are shown our experiences at welding of seam welds and circular welds of the cylindrical housing from superaustenitic stainless steel type 904L with thickness of 1 mm. Seam welds were done without filler

Figure 1: Schema for welding of longitudinal seam of cylindrical housings

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Figure 2: Typical view of starting place of the seam from the crown and root side. Cross-section geometry of the weld considerably effects on possible perforation/burn-through of the plate. As the valuation of examination results of the weld cross-sections can be mentioned that wide welds are against the protection from the perforation. Sometimes width of the welds from the crown side is three times larger than the plate thickness, and the weld width at the root side often reach the value of the thickness (Fig. 3). Thus the weld is too wide, height of weld bead is bigger than the crown height, which characteristically is below 0.1 mm. Burn-through is caused directly because of the collapsing of the too wide weld bead from the root side. If it is not the flat/plain/even and wide weld in comparison with the thickness means too large weld pool, which has disadvantages all from the corrosion and mechanical features. Endings of the seam welds are critical places at the welding of circular welds. Main places for damages at circular welds when it reaches the seam welds. One of the typical causes of this is the shape distortion and its consequence: a planar distortion, which generally cannot be extinguished easily by alignment (Fig. 4).

Figure 3: Etched metallographic pictures showing the cross section of cylindrical housing’s longitudinal seams; it is well demonstrated that a little misalignment of workpiece or torche have significant effect on the weld bead geometry

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Figure 5: Perfect and failed weld connections (on the top) and a typically failed circular weld (bottom)

Role of plasma torch accessories

Figure 4: Entry side of a longitudinal seam before hammering and after hammering

One of the most delicate/sensitive component of the welding system producing all the circular welds and the seam welds is the plasma-welding torch. Setting and servicing of which has grate importance. Operators usually know it well, but maintenance has to be connected to a condition-supervision system. Torch is a so-called dual flow torch, which can correct the movement of the weld shape forward the penetration direction against the widening (that is why so specific the presented too flat/plain/even weld shape). In Fig. 6 there are shown the weld shape characteristics when using TIG and plasma torches, and Fig. 7 schematizes the effect of electrode setback to the weld bead geometry.

Welding of circular welds For the fitting gap at circular welds by the side of cylindrical housing edge it is a prescription: components “must be fully pressed”. Nevertheless it often fail to devoid of gap, and in this case have to consider with burn-through of the weld in spite of the fact that the components welded to the housing are usually thick. As the circular shape defect is the largest around the seam weld, so here is the most typical the burnthrough. Burn-through characteristically means the burnthrough of the housing sheet, as the circular weld on some parts – mainly near the seam weld (Fig. 5) – does not lie on the joining cylindrical surface. Because of the gap the heat deposition is not good and the heat concentration can cause the burn-through of the overheated housing. If the fitting cannot be perfect, the welding parameters have to determine so that the weld have to be carried out safety.

Figure 6: Welding torches: TIG, plasma, dual flow plasma

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Role of the tungsten electrode In some case after removing the ceramic shielding nozzle it is put back if it is not necessary to change, but in this process the filter can become deformed, which can cause also faults in the gas flow. Inside the ceramic nozzle can be found the copper alloy liner and its tip (Fig. 9). This is the most important element of the constant working of plasma torch. Examined the changed nozzle we found that polluted with burr depositions, tungsten particles, and local melting points, rough overheating. Particularly important to determine by experiments the changing periods, optimal cycle time period of this component. Shape of the tip of tungsten electrode at plasma arc welding and plasma beam welding has much more importance in comparison with TIG welding: the condition of tungsten electrode considerably effects on the quality of plasma beam and weld.

Figure 7: Influence of the tungsten electrode position on the shape of plasma and weld bead geometry External component of the torch is the ceramic nozzle (Fig. 8), which with the inside gas lens is suited to protect laminar protective gas flow. In some cases woven filter of the gas lens gets roughly filled and it can strongly divert the transferred plasma too; in this case the pilot arc flickers, crackles. In the gas lens sometimes there are rough pollutions and obstructions, because of which necessary to find the pollution source and if it is possible to eliminate it.

Figure 8: Ceramic nozzle of the PWM-4A type torch

Figure 9: Liner and tip of the PWM-4A plasma torch

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Tungsten electrode tip usually is pointed by grinding (Fig. 10), but the application of tungsten electrode grinder is not at all general. Use of tungsten electrode grinder is general in factories working with plasma beam welding, but consideration of grain size of the grinding disc is very rarely.

Figure 10: Tip of tungsten electrodes before welding

Figure 11: Tip of tungsten electrodes after welding Perhaps the blunting of electrode tip is still less characteristically however the sharp tip is the most sensible part of the electrode, and as it is shown in Fig. 11 there is the strongest the wear. Plasma beam from the wear electrode is instable, heat input can change and therefore the danger of burningthrough of the weld is bigger. Scanning electron microscopic pictures of Fig. 12-13 also show the typical wear.

Figure 12: Wear of tungsten electrode tips

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Figure 14: Well-sharpened tungsten electrode tip for plasma arc welding with dual flow plasma torch; roughness of blunted surface after coarse (bottom left) and fine grinding (on bottom right).

Summary The role of tungsten electrode at plasma arc welding is dual: on the one hand it effects on geometrical features of the weld with the position in plasma-welding torch, on the other hand depending on shape of the tip it can effect on stability of plasma arc or plasma beam. By what means Fig. 14, it is absolutely necessary to blunt the electrode tip to value 0.5 mm, roughness < 5-1 μm.

Acknowledgements This work was supported in part by Hungarian Scientific Research Found (grant: NKTH–OTKA K61922).

References [1] [2] [3] [4] [5]

Figure 13: Wear of tungsten electrode tips

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